The dsDNA bacteriophage HK97 is a member of the Siphovirus (lambdoid) family. The assembly pathway has been determined and many of the individual steps can be carried out in vitro, making this an excellent model system to study viral capsid assembly. Although dsDNA phage have been extensively studied, a high resolution structure has not been determined; such a structure is required to understand the phage assembly mechanism in greater detail. We have crystallized the Head II capsid, the final step in the assembly pathway. The crystals diffract x-rays to at least 3.5_ using synchrotron radiation. The unit cell is monoclinic, with dimensions a=580_, b=625_, c=787_ b = 90.0 degrees, and two particles per cell. Systematic absences of even reflections along the 0k0 lattice line indicate that the space group is P21. Data to 5_ were collected at CHESS; this resolution limit is due to the need to position the detector at a sufficient distance to resolve the diffraction maxima. Approximately 250 images were processed and scaled, resulting in a data set with a cumulative completeness of 65%, and a scaling R factor of 7.7% to 7_. The rotation function was used to determine the orientation of the particles in the unit cell. An icosahedral 2-fold axis is approximately, but not exactly, aligned with the crystallographic 21 axis (b-axis). The orthogonal icosahedral 2-fold is rotated 18 degrees away from the a axis. The two particles must be positioned close to their ideal packing arrangement, which places one particle at (1/4, y, 1/4) and the other particle at (3/4, y, 3/4). The particles interact closely along the icosahedral 3-fold axes; this occurs both along the crystallographic a axis, and along the body diagonal of the unit cell. The packing interactions agree well with the particle dimensions determined by cryoEM reconstruction (Conway, et al, 1995, Proteolytic and conformational control of virus capsid maturation: The bacteriophase HK97 System, J. Mol. Bio. 253:86-99). This cryoEM reconstruction will be used as a model to initiate phase determination; structure determination at 7 A is underway.